1. Field of the Invention
The invention relates to a method and apparatus for monitoring a flame state in a combustion chamber of a gas turbine using dynamic pressure sensors. More specifically, it relates to the use of a dynamic pressure sensor arranged in a pressure influence zone in a combustor of the combustion chamber, and comparing a spectral pattern of the sensor output signal to a characteristic frequency pattern that includes information about an acoustic pattern of the flame and information about acoustic signal canceling due to reflections within the combustor.
2. Description of the Prior Art
A gas turbine is a flow machine in which a pressurized gas expands. It comprises a turbine or expander, a compressor connected upstream thereof, and a combustion chamber positioned therebetween. The operating principle is based on the cycle process (Joule process): This compresses air by way of the blading of one or more compressor stages, subsequently mixes said air in the combustion chamber with a gaseous or liquid fuel, ignites and combusts the same. The air is also conducted into a secondary air system and utilized for cooling in particular components that are subject to extreme thermal stresses.
This results in a hot gas (mixture composed of combustion gas and air) which expands in the following turbine section, with thermal energy being converted into mechanical energy in the process and in the first instance driving the compressor. The remaining portion is employed in the shaft driving mechanism for driving a generator, a propeller or other rotating loads. In the case of the jet power plant, on the other hand, the thermal energy accelerates the hot gas stream, which generates the thrust.
Typically, a plurality of combustors is provided, arranged annularly around the turbine axis and having corresponding injector nozzles for fuel. In such a configuration the combustors can be arranged as individual combustors, referred to as baskets, which are connected only shortly before the entry into the turbine (referred to as a can or can-annular design), or the combustors can be arranged in a common ring-shaped combustion chamber (referred to as an annular design). When the gas turbine is started up, the fuel-air mixture in the respective combustion chamber is ignited by means of igniters. Thereafter the combustion takes place continuously.
The continuous monitoring of the flame, in particular in each individual combustor in the case of the can-type or can-annular-type design, is important for the operational safety of the gas turbine in order to avoid dangerous situations due to the ingress of unburnt fuel in the combustion chamber or the turbine outlet. In this case the monitoring of the flame state must be performed quickly so that no dangerous air-fuel mixtures are produced over a relatively long period of time. Response times of less than a second are desirable. In particular the igniting and extinguishing of flames must be reliably detected at any given time, especially also in situations such as a load throw-off, during powering down of the gas turbine, or in partial extinguishing of individual flames.
Optical and temperature-based systems are known for monitoring the flame state. Optical systems measure the light emitted by the flame directly and typically are comparatively quick. A disadvantageous aspect with systems of said type, however, is the susceptibility of the optical components to soiling by particles, dust, soot, oil, as well as water and condensation. The soiling reduces the flame detection capabilities of such systems as well as their reliability and operational availability.
As an alternative to such optical systems, systems have therefore been developed which are based on the dynamic measurement of the pressure in the pressure influence zone. A system of said type is described for example in U.S. Pat. No. 7,853,433 B2. In this case a piezoelectric pressure sensor is arranged in the pressure influence zone of each combustor. The time signal of the pressure sensor is digitized and subjected to a wavelet analysis. The wavelet analysis enables the flame state and a flame flashback to be detected based on the comparison of the normalized amplitudes of the wavelet coefficients with predetermined threshold amplitudes. In this case the signals are normalized using the mean value of all of the combustors, as a result of which the threshold value is specified. If said threshold values are exceeded it signifies a deviation from the normal state and consequently a change in the flame state, either the igniting or extinguishing of the flame or a flame flashback.
However, the method described in U.S. Pat. No. 7,853,433 B2 has the disadvantage that certain flame states are not detected. An extinction of all of the combustor flames will not be detected, for example.
To reduce costs it is desired to use already existing sensors for flame monitoring. Dynamic pressure sensors are available from the monitoring of the combustion dynamics and can be used for flame monitoring. Some current combustion dynamics monitoring systems utilize two dynamic pressure sensors per combustor. Future gas turbines will potentially utilize only one dynamic pressure sensor per combustor. To prevent the need for additional instrumentation and thus to keep a cost advantage, a need exists in the art to detect and monitor a turbine combustor flame using a single sensor per combustor.
A further need exists in the art to detect a condition of simultaneous flame-out in all combustors using a single sensor per combustor.
There is an additional need in the art to filter the acoustic data received from a sensor in a combustor to focus the analysis on specific, localized sound sources while disregarding background noise.